The EMP Grenade can also be found in Zombies. More specifically, it's found in the Mystery Box on all Green Run related maps. The EMP renders the zombies in 'Sleep mode' exactly the same as the beginning of Green Run.

They will stay in this way if the player or players

Don't shoot any guns.

Don't explode any grenades.

Don't run.

Don't Touch them.

If a player fires gun, the zombies could 'wake-up' and resume attacking faster then a player can recover from the daze of the EMP Grenade.